1. Field of the Invention
This invention relates to a bridge inverter for use in induction heating apparatuses having large load variations, particularly induction heating cooking appliances.
2. Description of the Prior Art
Generally, in an inverter device for induction heating cooking appliances, since the load is in the form of a pan, stabilized operation is required irrespective of the material of the pan and irrespective of the presence or absence of a pan. Further, as is known in the art, a bridge inverter comprises a plurality of series-connected switching elements connected to a power source, the output from said converter being obtained at the junction of the series connection, said switching elements being alternately or successively driven. This inverter, however, has drawbacks; for example, when the switching time of the elements is prolonged by a temperature increase or when there is a large variation in the load, there is the danger of the switching elements being simultaneously rendered conductive and thereby damaged. As a solution to this problem, the common means is to provide a fixed dwell period for stopping all the switching elements at the drive signal switching time in consideration of the amount of variation when the switching time varies. This means, however, does not essentially eliminate the danger of simultaneous conduction, and the provision of a sufficient dwell period has been the major cause of reduction of the operating efficiency of the inverter device. On the other hand, when an erroneous input signal is transferred to the control circuit, this is very undesirable since it leads to the simultaneous conduction of the switching elements of the inverter device or to abnormal oscillation thereof if the signal is on the level of not damaging the elements. As a solution of this problem, the common means is to stabilize the circuit by using a capacitor or the like which bypasses erroneous input signals. This means, however, depends on the correlation between the capacitance of the capacitor and the magnitude of the erroneous input signal, and can hardly serve as a radical solution to the problem.